feat(k8s): helmfile backend for !kubectl (CLI) (#43)

[Zorlin]

Part of #43. Depends on the !kubectl core (#68).

What

The helmfile backend: helmfile: source → helmfile, via the helmfile CLI.

• state: present → helmfile apply / sync; absent → helmfile destroy; fetched → helmfile status; dry_run: true → helmfile template.
• save: exposes the rendered output.

Anchors

• src/modules/commands/kubernetes/helmfile.rs (new) — implements KubernetesBackend via the helmfile CLI subprocess.
• The helmfile source branch is already accepted by infer_source (chunk 1).

Tests

• Unit (no cluster): helmfile CLI argv construction per state×source.
• Integration (cluster): sync a helmfile → releases deploy; destroy removes them.